For purposes of clarity, the following definition is offered for the term “split-cycle engine” as may be applied to engines disclosed in the prior art and as referred to in the present application.
A split-cycle engine as referred to herein comprises:
a crankshaft rotatable about a crankshaft axis;
a compression piston slidably received within a compression cylinder and operatively connected to the crankshaft such that the compression piston reciprocates through an intake stroke and a compression stroke during a single rotation of the crankshaft;
an expansion (power) piston slidably received within an expansion cylinder and operatively connected to the crankshaft such that the expansion piston reciprocates through an expansion stroke and an exhaust stroke during a single rotation of the crankshaft; and
a crossover passage interconnecting the compression and expansion cylinders, the crossover passage including a crossover compression (XovrC) valve and a crossover expansion (XovrE) valve defining a pressure chamber therebetween.
U.S. Pat. No. 6,543,225 granted Apr. 8, 2003 to Carmelo J. Scuderi contains an extensive discussion of split-cycle and similar type engines. In addition, the patent discloses details of a prior version of an engine of which the present invention comprises a further development.
Referring to FIG. 1, an exemplary embodiment of the prior art split-cycle engine concept is shown generally by numeral 10. The split-cycle engine 10 replaces two adjacent cylinders of a conventional four-stroke engine with a combination of one compression cylinder 12 and one expansion cylinder 14. These two cylinders 12, 14 perform their respective functions once per crankshaft 16 revolution. The intake air and fuel charge is drawn into the compression cylinder 12 through typical poppet-style intake valves 18. The compression cylinder piston 20 pressurizes the charge and drives the charge through the crossover passage 22, which acts as the intake passage for the expansion cylinder 14.
A check type crossover compression (XovrC) valve 24 at the crossover passage inlet is used to prevent reverse flow from the crossover passage 22 into the compression cylinder 12. A crossover expansion (XovrE) valve 26 at the outlet of the crossover passage 22 controls flow of the pressurized intake charge such that the charge fully enters the expansion cylinder 14 shortly after the expansion piston 30 reaches its top dead center (TDC) position. Spark plug 28 is fired soon after the intake charge enters the expansion cylinder 14 and the resulting combustion drives the expansion cylinder piston 30 down toward bottom dead center (BDC). Exhaust gases are pumped out of the expansion cylinder through poppet exhaust valves 32.
With the split-cycle engine concept, the geometric engine parameters (i.e., bore, stroke, connecting rod length, compression ratio, etc.) of the compression and expansion cylinders are generally independent from one another. For example, the crank throws 34, 36 for each cylinder may have different radii and be phased apart from one another with top dead center (TDC) of the expansion cylinder piston 30 occurring prior to TDC of the compression cylinder piston 20. This independence enables the split-cycle engine to potentially achieve higher efficiency levels and greater torques than typical four stroke engines.
Because the crossover expansion (XovrE) valve 26 has only a short time (about 30 degrees crank angle) to discharge pressurized air/fuel mixture into the expansion cylinder prior to completion of the compression cylinder piston stroke, closing of the crossover expansion valve occurs after ignition of the air/fuel charge. It is desired for extended valve life to avoid entry of burning fuel mixture into the crossover expansion valve without shortening the valve closing timing.